<select name="zona" id="zona">
  <option value=""></option>
  <option value="Centro" <? if($zonaSeleccionada == "Centro") echo "SELECTED";?>>Centro</option>
  <option value="Norte" <? if($zonaSeleccionada == "Norte") echo "SELECTED";?>>Norte</option>
  <option value="Oriente" <? if($zonaSeleccionada == "Oriente") echo "SELECTED";?>>Oriente</option>
  <option value="Poniente" <? if($zonaSeleccionada == "Poniente") echo "SELECTED";?>>Poniente</option>
  <option value="Sur" <? if($zonaSeleccionada == "Sur") echo "SELECTED";?>>Sur</option>
</select>